1. Field of the Invention
This invention relates to a semiconductor circuit.
2. Related Background Art
For example, a current switch in a semiconductor integrated circuit is often used in A/D and D/A converters now existing as the interface between the analog and the digital signals. One of A/D and D/A converters is comprised of a constant current circuit and a resistor circuit, and the current switch is used for switching the current of the constant current circuit relative to said resistor circuit.
Operation of the current switch will hereinafter be described on the basis of the conventional circuit of FIG. 1 of the accompanying drawings. Referring to FIG. 1, reference numerals 11-14 designate lateral PNP type transistors (hereinafter referred to as L-PNP) for a constant current. Reference numerals 7-10 denote resistors for determining the value of the current, and reference numerals 15-21 designate ladder type resistor networks. Reference numerals 22-25 denote NPN transistors which provide current switches and which effect the switching operation by a voltage being supplied to the base inputs thereof corresponding to each bit of a digital value. The base-emitter voltage of L-PNP 11-14 is controlled by the switching operation to discretely vary the currents flowing into the resistor networks. As a result, an analog voltage corresponding to the digital value is obtained at a terminal 4.
However, this circuit has the following disadvantages:
(1) Power consumption is increased by the switching operation. The base-emitter voltage V.sub.BE of L-PNP 11-14 becomes extinct due to the switching of the NPN transistors 22-25 and therefore, a current of V.sub.BE /R8 increases.
(2) A large current flows into the bias line 5 of L-PNP 11-14 and therefore, the current driving capability of the bias circuit 1 must be made great.
(3) Since the current flowing through the bias line 5 of L-PNP 11-14 is great, the influence of the voltage drop in the bias line cannot be neglected.
(4) Since the current flowing into the ground line 3 varies, the amount of voltage drop in the ground line varies, and this leads to the undesirable possibility that the ground line level fluctuates.